Glass-ceramic articles have been marketed commercially for over 30 years. The initial disclosure of such articles is found in U.S. Pat. No. 2,920,961 (Stookey). As is explained in that patent, glass-ceramic articles are prepared via the heat treatment of precursor glass bodies. Thus, producing glass-ceramic articles comprehends three general steps: (1) a glass forming batch customarily containing a nucleating agent is melted; (2) that melt is cooled to a temperature below the transformation range thereof and, concurrently with said cooling, a glass article of a desired geometry is shaped therefrom; and (3) that glass article is exposed to a heat treatment schedule to crystallize the glass in situ. Commonly, the heat treatment involves two stages; viz., the glass article is initially heated for a time at a temperature somewhat above the transformation range to cause the generation of nuclei therein after which the temperature is raised to above the annealing point and, frequently, in excess of the softening point of the glass to effect the growth of crystals on the previously-generated nuclei. The process yields products which are typically highly crystalline, the crystals being fine-grained, substantially uniform in size, and homogeneously distributed throughout the minor amount of residual glassy phase.
Whereas Pat. No. 2,920,971 is directed to the preparation of opaque glass-ceramic articles and the sales of opaque glass-ceramic articles have far exceeded those of transparent glass-ceramic articles, there has been a large market for transparent glass-ceramic articles, one such product being cookware marketed by Corning Incorporated, Corning, New York under the trademark VISIONS.RTM.. That product is manufactured under U.S. Pat. No. 4,018,612 (Chyung), which patent covers compositions within a narrow range of the Li.sub.2 O-MgO-ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 system nucleated with a combination of TiO.sub.2 and ZrO.sub.2 to yield highly crystalline articles containing .beta.-quartz solid solution as the primary crystal phase.
Recent marketing studies have indicated the desirability of developing a transparent glass-ceramic body suitable for cookware exhibiting a soft blue tint. U.S. Pat. No. 4,018,612 notes the possible inclusion of glass colorants in the original glass composition, specifically referring to cobalt, chromium, copper, iron, manganese, nickel, and vanadium U.S. Pat. No. 4,192,688 (Babcock et al.) also discusses the use of glass colorants in glass-ceramic articles. The latter patent points out that the colors produced in a glass-ceramic body are frequently quite different from the color present in the precursor glass body. And the color of a transparent glass-ceramic body employing the same coloring agent will frequently be different from that displayed in an opaque glass-ceramic body. To illustrate, cobalt has long been recognized as imparting a blue color to glass, but in a transparent glass-ceramic the color becomes lavender and in an opaque glass-ceramic the color is a very pale blue. In another example a combination of cobalt and nickel imparts an amber color to the parent glass article, but the transparent glass-ceramic article derived therefrom exhibits a cranberry red hue and the opaque glass-ceramic body produced therefrom a blue shade.
The production of colored transparent glass-ceramic articles containing .beta.-quartz solid solution as the primary crystal phase from precursor glasses having base compositions within the lithium aluminosilicate system nucleated with TiO.sub.2 is described in U.S. Pat. No. 4,084,974 (Beall et al.). As is disclosed there, a carbonaceous reducing agent (expressly discussing the utility of carbon, starch, and sucrose) is included in the glass forming batch such that, during melting, some of the Ti.sup.+4 ions will be reduced to Ti.sup.+3 ions Thereafter, when the glass body is heat treated to crystallize in situ the resulting transparent glass-ceramic will exhibit a blue-to-black color.
Whereas the operability of that procedure was evidenced repeatedly on a small laboratory scale, it proved difficult to control in large scale commercial production. Thus, problems in fining of the glass melt were encountered and variations in physical properties such as the coefficient of thermal expansion were witnessed.
Therefore, the principal objective of the instant invention was to develop transparent glass-ceramic articles containing .beta.-quartz solid solution as the predominant crystal phase exhibiting a linear coefficient of thermal expansion (25.degree.-600.degree. C.) less than 10.times.10.sup.-7 /.degree. C., preferably less than 6.times.10.sup.-7 /.degree. C., and displaying a soft blue coloration without the use of a carbonaceous reducing agent.